Since I don't have the full context of the values within your tableView method I can only speculate. That said, the sender you're passing in should be the view controller:

performSegue(withIdentifier: "toGuestVC", sender: username)

You're passing in a value called username which looks to be a string value? It should be something like:

performSegue(withIdentifier: "toGuestVC", sender: self)

where self is your view controller. If you're passing in a string value to sender then in your prepareForSegue method then sender does not have a property called username sender actually isusername. Therefore you should pass the value elsewhere: